Alan,

I believe that you are describing a situation where the Onan attempts to turnover, but it seems as though you are trying to start it with a weak or dead battery. I recently reviewed this situation with a client. I think this may help, if not solve the issue:

On all of the SMs and 54s that I have checked, the length of battery cable to the Onan is almost twice as long as the cable to main engine, but the size of the wire to the Onan is the same size (gauge) as the main engine. Logic tells me that either the main engine battery cable is oversized, or the Onan cable is undersized.

The Onan has starter battery connection terminals on the left-side-facing and near the rear of the Onan. The Onan has a hidden-from-view cables from the battery connection terminals mentioned above to the battery side of the negative solenoid and positive starter solenoid. This is Onan Part Number: 338-3577-01 Battery Harness. 

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I believe, that in at least some cases, this  Onan Part Number 338-3577-01 Battery Harness  is the cause of a loss of amperage throughput to the starter. I have advised a few clients when experiencing this issue to:

1. Ensure that the starting battery is in good condition 
2. The battery wire terminal blocks in the first sentence above are clean and treated with CorrosionX
3. The negative solenoid is working correctly
4. And that all starting circuit and battery connections are clean and treated with CorrosionX

If all of the above is checked/completed and the Onan is still not starting, You should shut off Battery Main Switches to perform the following:  

1. Fabricate a jumper wire using appropriate sized copper wire and ring terminals (probably at least 8mm diameter (AWG-1)) from the negative connection block on the outside of the Onan (mentioned above) to the battery-side of the negative solenoid. 
2. If this does not work, next fabricate a jumper for the positive side. 

IMPORTANT TO ALL: If you cannot identify the components I have identified and/or you are not competent with DC voltage and wiring, seek professional help to do this.

Alan, I know that you are competent. Please let me know if this works for you and I would appreciate photos. I have tried to find a drawing of the suspected connection wire, but have not. There is limited evidence that points to this jumper wire being a solution, but I am not positive.

Best,

CW Bill Rouse
Yacht School - Supporting Amel Owners
www.YachtSchool.us
720 Winnie St
Galveston Island, TX 77550
+1(832) 380-4970

On Sat, Aug 10, 2019, 7:57 PM Alan Leslie <s.v.elyse@...> wrote:

I know this is an old thread, but it serves to indicate how useful this forum is.
I had the issue of the Onan not cranking over the last few days.
Press the start button and it would prime, it would try to crank and then stop.
I checked all the power connections, cleaned and retightened...same issue.
then I went in the forum and saw this post.
Hmmm...so I started the main engine, and the Onan cranked no problem.
So it's the start battery?.no, well it is and it isn't.
I hadn't been on the boat for 4 months but i have a small 12v maintenance charger to keep the start battery up.
And that's what the problem is - the charger had died!
I've since charged up the start battery (1008CCA) and load tested it (perfect) and the Onan cranks every time.
So big thank you to everyone on the forum for all the input, it truly is a marvelous resource.
Cheers

Alan
Elyse SM437

Thanks Bill,
That all makes sense and since I've fully charged the start battery, the ONAN cranks every time.
I had been through ALL the connections including the hidden from view ones (you don't really want to get in there !) disassembled, cleaned and treated with CorrosionX.
I even put the negative lead directly to the crankcase, still no good.
But it was Mark's thread about starting the main engine that led me to it.
My little maintenance charger had died and the start battery was down a bit - I should have checked that first, so I'm not that competent !
I agree with you, there must be quite a volt drop along those long cables that feed the ONAN starting system, as you say, undersized.  
BUT with a fully charged start battery, it's not an issue.

We learn things every day.

However this is not the end of the story. While the ONAN cranks it only runs for 2 seconds and shuts down with error code 12, overvoltage.
I had the Cummins man here today and he's visually checked the diodes and condition of the windings - all looks OK, and he will come back with the tester to test the rotor and stator windings to be sure they are within spec, and if they are, he says its a fault on the control board...which means replacing the board...hmm

Cheers
Alan
Elyse SM437